As an exhaust gas purification system for purifying NOx in exhaust gas of a diesel engine, an SCR (Selective Catalytic Reduction) system using an SCR device has been developed.
The SCR system supplies urea water to an exhaust gas upstream of SCR, generates ammonia by the heat of the exhaust gas, and reduces and purifies NOx on an SCR catalyst by the ammonia (see, for example, Patent Document 1).
In the SCR system, a temperature of urea water stored in a urea tank is measured by a urea water temperature sensor provided in the urea tank. Thus, freezing of the urea water is detected, and thawing control is performed as necessary.
Since the temperature of the urea water cannot be measured when the urea water temperature sensor fails, the SCR system includes a competence diagnosis system for a urea water temperature sensor that diagnoses a failure of the urea water temperature sensor.
The competence diagnosis system diagnoses a failure of the urea water temperature sensor by comparing a temperature of the urea water measured by the urea water temperature sensor with an ambient temperature. Specifically, the competence diagnosis system compares a temperature of the urea water with an ambient temperature immediately after engine starting, and diagnoses the urea water temperature sensor as failed when a difference between the temperatures is more than a predetermined threshold value. Since there is no temperature sensor that directly measures an ambient temperature to be compared, for example, an intake manifold temperature or an MAF (Mass Air Flow) temperature measured by an MAF temperature sensor provided in an MAF sensor is used as the ambient temperature.
The reason for performing the diagnosis immediately after the engine starting is that it is expected that a temperature of the urea water in the urea tank and an ambient temperature would be substantially the same immediately after the engine starting, and a failure of the urea water temperature sensor can be diagnosed by determining whether or not there is a significant difference between these temperatures.
In the conventional competence diagnosis system, the diagnosis is allowed in only a predetermined period after the engine starting so that the diagnosis is performed only immediately after the engine starting.